Bistatic radar principals allow a passive operator to locate a target utilizing both reflected and direct signal emissions generated from a third source. Advantageously, emissions from a non-cooperative radar transmitter are utilized to illuminate a target, and the reflected emissions from the target are processed by a receiver to locate the position of the target. A non-cooperative radar may constitute an enemy radar in a wartime environment, or simply a radar transmitter which is not under the control of the receiver. The procedure of utilizing a non-cooperative radar to illuminate a target has the advantage of locating a target without emitting signals from the searching body thereby avoiding transmissions which can be used to reveal the location of the searcher. This ability to use signals propagated from a non-cooperative third source to avoid detection yet locate target objects has obvious advantages in a hostile environment.
In U.S. Pat. No. 3,812,493--by Afendykiw, et al, a bistatic passive radar was disclosed wherein a pair of interferometer antennas were utilized to measure the time delay in receiving a reflected signal from a target ascompared to receiving a direct signal from a signal source; the time delay was then processed to calculate the distance between the target and the receiver. By cross correlating the phase difference between the same set of signals received at two spaced apart interferometer antennas, the angle of arrival of the signals was determined, which together with the target range was used to locate the position of the target.
Another approach for locating the positions of objects reradiating transmitted energy was disclosed in U.S. Pat. No. 2,971,190--by Busignies, wherein a receiver measured the time difference between receipt of signals directly from a transmitter and signals generated from the transmitter and reradiated from other target objects. A direction finder/receiver utilizing a rotatable directive antenna driven at a predetermined speed generated data indicating the angle of the reradiated signals and the relative timing between the instant when the transmitter was aimed at the receiver and when the transmitter was aimed toward the reradiating object. The resulting values in combination with the time difference between the receipt of direct and reradiated signals was used to locate the position of the target objects with respect to the receiver.
An array of three or more receiving antennas located in a triangular relationship was disclosed in U.S. Pat. No. 4,393,382--by Jones, to determine the range between a transmitter of RF energy and the antenna array, wherein the various measured time intervals for the source transmitter to sweep through angles subtended by the receiving antennas together with the scan rate of the transmission source was utilized to determine the angle of arrival of the transmissions, which were further combined with a measurement of the sweep time differential for a selected pair of the arrayed antennas to generate a range between the transmitter and the receiving antennas.
In U.S. Pat. No. 4,173,760--by Garrison, apparatus and methods were disclosed for identifying the location of a jamming radar target by generating reference pulses of varying frequency which are utilized in determining the phase relationship between the reference pulses and the corresponding frequency components of the jamming signal as received over a direct path between the reference and the target, and over an indirect path from a cooperative auxiliary receiving antenna spaced a known distance from the primary receiving antenna. The phase difference between the direct and indirect received signals were then combined to locate the jamming transmitter.
The location of a source transmitter is determined using an angle measuring interferometer system in U.S. Pat. No. 3,935,574--by Pentheroudakis, wherein the interferometer utilized a longitudinal antenna array having a base line length greater than the signal wavelength to generate accurate phase measurements which are indeterminate as to the number of cycles which occur between the signal's time of arrival at the two antennas. The indeterminate phase measurements were resolved by generating all possible bearings corresponding to the phase measurement and then utilizing a tracking procedure to establish the correct signal source position.
In U.S. Pat. No. 4,370,656--by Frazier, et al, a bistatic passive radar system is utilized for determining the distance between a receiving aircraft and a transmitting aircraft utilizing radar signals generated from the transmitting aircraft directly to the receiving aircraft as well as transmitted signals reflected from a selected ground location between the two aircraft. Utilizing data such as the altitude of the receiving aircraft, the angle with respect to the vertical which the radar signals are received directly from the transmitting aircraft, the angle with respect to the vertical at which the reflected signals from the ground are received, and an apparent range from the transmitter to the receiver, the distance between the aircraft are computed.
In U.S. Pat. No. 3,210,762--by Brabant, a method or determining the range and altitude of an object which radiates or reflects electromagnetic energy from a receiving object is disclosed wherein variables such as the path length difference between the direct path and the reflective path, a grazing angle at which radiation is reflected from the surface, and the altitude of the receiver are utilized to determine the range between the aircraft. Range, bearing an elevation information to a transmitter of electromagnetic radiation is obtained through ratio comparison of signals received at detectors located at opposite wing tips as well as the nose and underside of an aircraft, in a manner that distance information is determined by measuring the intensities of energy received by respective detectors, and whereas bearing information is determined by orienting the heading of the aircraft with respect to the transmitting source or with respect to an object reradiating reflected energy from the transmitting source.
Another method for determining the range between an emitting target aircraft and a detecting receiving aircraft is disclosed in U.S. Pat. No. 3,721,986--by Kramer, wherein the range between a target aircraft and a receiving aircraft is determined by measuring the angle of elevation between the two aircraft, the altitude of the detecting aircraft above the terrain, and the time interval between arrival at the detecting aircraft of electromagnetic wave energy transmitted from the target aircraft directly to the detecting aircraft, and transmitted to a reflecting surface which is then received by the detecting aircraft.
In U.S. Pat. No. 3,789,410--by Smith, et al, wherein the rate of change in phase difference between two signals received by separated pairs of antenna on a receiving aircraft are utilized to determine the range between a target aircraft and the receiving aircraft.
Radiation from a celestial source such as the sun which is reflected from a target and picked up by a scanning search antenna is compared in Wiley, U.S. Pat. No. 3,171,126, with radiation obtained directly from the sun and received by a receiving reference antenna closely mounted to the scanning search antenna, and the time differential therebetween utilized to determine the target range.
An improvement to the apparatus and methods of U.S. Pat. No. 4,339,755 is discussed in an application Ser. No. 537,498 entitled "PASSIVE RANGING METHOD AND APPARATUS USING INTERFEROMETRIC SCANNING" by Fred M. Lightfoot, filed Sept. 30, 1982; and assigned to the Assignee of the present invention.